mnt: tune the root mount before mounting a root yard

Otherwise the root yard can be propagated into the host mount namespace
and remain there and criu will fail, because it will not be able to
remove the roots yard.

It occures if we give a shared mount as root to "criu restore" and
criu converts it into a slave mount.

Reported-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>